The present invention relates to an inert gas purge method which is preferably applied to an exposure apparatus that uses ultraviolet rays as exposure light, purges the interior of the apparatus with inert gas, and projects the pattern of a master such as a mask onto a photosensitive substrate via a projection optical system, and which purges with inert gas a pellicle space defined by a master and a pellicle used to prevent deposition of a foreign matter on a pattern surface. The present invention also relates to an exposure apparatus having an inert gas purge apparatus for purging the pellicle space with inert gas. The present invention further relates to a reticle stoker, reticle inspection apparatus, reticle transfer box, and device manufacturing method using the inert gas purge apparatus.
A conventional manufacturing process for a semiconductor element such as an LSI or VLSI formed from a micropattern uses a reduction type projection exposure apparatus for printing and forming by reduction projection a circuit pattern drawn on a master such as a mask onto a substrate coated with a photosensitive agent. With an increase in the packaging density of semiconductor elements, demands have arisen for further micropatterning. Exposure apparatuses are coping with micropatterning along with the development of a resist process.
A means for increasing the resolving power of the exposure apparatus includes a method of changing the exposure wavelength to a shorter one, and a method of increasing the numerical aperture (NA) of the projection optical system.
As for the exposure wavelength, a KrF excimer laser with an oscillation wavelength of 365-nm i-line to recently 248 nm, and an ArF excimer laser with an oscillation wavelength around 193 nm have been developed. A fluorine (F2) excimer laser with an oscillation wavelength around 157 nm is also under development.
An ArF excimer laser with a wavelength around ultraviolet rays, particularly, 193 nm, and a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm are known to have a plurality of oxygen (O2) absorption bands around their wavelength bands.
For example, a fluorine excimer laser has been applied to an exposure apparatus because of a short wavelength of 157 nm. The 157-nm wavelength falls within a wavelength region generally called a vacuum ultraviolet region. In this wavelength region, light is greatly absorbed by oxygen molecules, and hardly passes through the air. The fluorine excimer laser can only be applied in an environment in which the atmospheric pressure is decreased to almost vacuum and the oxygen concentration is fully decreased. According to reference xe2x80x9cPhotochemistry of Small Moleculesxe2x80x9d (Hideo Okabe, A Wiley-Interscience Publication, 1978, p. 178), the absorption coefficient of oxygen to 157-nm light is about 190 atmxe2x88x921 cmxe2x88x921. This means when 157-nm light passes through gas at an oxygen concentration of 1% at one atmospheric pressure, the transmittance per cm is only
T=exp(xe2x88x92190xc3x971 cmxc3x970.01 atm)=0.150
Oxygen absorbs light to generate ozone (O3), and the ozone promotes absorption of light, greatly decreasing the transmittance. In addition, various products generated by ozone are deposited on the surface of an optical element, decreasing the efficiency of the optical system.
To prevent this, the oxygen concentration in the optical path is suppressed to low level of several ppm order or less by a purge means using inert gas such as nitrogen in the optical path of the exposure optical system of a projection exposure apparatus using a far ultraviolet laser such as an ArF excimer laser or fluorine (F2) excimer laser as a light source.
In such an exposure apparatus using an ArF excimer laser with a wavelength around ultraviolet rays, particularly, 193 nm, or a fluorine (F2) excimer laser with a wavelength around 157 nm, an ArF excimer laser beam or fluorine (F2) excimer laser beam is readily absorbed by a substance. The optical path must be purged to several ppm order or less. This also applies to moisture, which must be removed to ppm order or less.
For this reason, the interior of the exposure apparatus, particularly, the optical path of ultraviolet rays is purged with inert gas. A load-lock mechanism is arranged at a coupling portion between the inside and outside of the exposure apparatus. When a reticle or wafer is to be externally loaded, the interior of the exposure apparatus is temporarily shielded from outside air. After the impurity in the load-lock mechanism is purged with inert gas, the reticle or wafer is loaded into the exposure apparatus.
FIG. 1 is a schematic sectional view showing an example of a semiconductor exposure apparatus having a fluorine (F2) excimer laser as a light source and a load-lock mechanism.
In FIG. 1, reference numeral 1 denotes a reticle stage for setting a reticle bearing a pattern; 2, a projection optical system for projecting the pattern on the reticle onto a wafer serving as a photosensitive substrate; 3, a wafer stage which supports the wafer and is driven in the X, Y, Z, xcex8, and tilt directions; 4, an illumination optical system for illuminating the reticle with illumination light; 5, a guide optical system for guiding light from the light source to the illumination optical system 4; 6, a fluorine (F2) excimer laser serving as a light source; 7, a masking blade for shielding exposure light so as not to illuminate the reticle except for the pattern region; 8 and 9, housings which cover the exposure optical path around the reticle stage 1 and wafer stage 3, respectively; 10, a He air-conditioner for adjusting the interiors of the projection optical system 2 and illumination optical system 4 to a predetermined He atmosphere; 11 and 12, N2 air-conditioners for adjusting the interiors of the housings 8 and 9 to a predetermined N2 atmosphere; 13 and 14, reticle load-lock chambers and wafer load-lock chambers used to load a reticle and wafer into the housings 8 and 9, respectively; 15 and 16, a reticle hand and wafer hand for transferring the reticle and wafer, respectively; 17, a reticle alignment mark used to adjust the reticle position; 18, a reticle temporary stocker for stocking a plurality of reticles in the housing 8; and 19, a pre-alignment unit for pre-aligning the wafer. If necessary, the overall apparatus is stored in an environment chamber (not shown). Air controlled to a predetermined temperature is circulated within the environment chamber to keep the internal temperature of the chamber constant.
FIG. 2 is a schematic sectional view showing another example of the semiconductor exposure apparatus having a fluorine (F2) excimer laser as a light source and a load-lock mechanism. In FIG. 2, the same reference numerals as in FIG. 1 denote the same parts.
The whole exposure apparatus shown in FIG. 2 is covered with a housing 20, and O2 and H2O in the housing 20 are purged with N2 gas. Reference numeral 21 denotes an air-conditioner for setting the entire housing 20 in an N2 atmosphere. In this exposure apparatus, the lens barrel of a projection optical system 2 and the internal space of an illumination optical system 4 are partitioned from the internal space (driving system space) of the housing 20, and independently adjusted to a He atmosphere. Reference numerals 13 and 14 denote a reticle load-lock chamber and wafer load-lock chamber used to load a reticle and wafer into the housing 20, respectively.
In general, a reticle is equipped with a pattern protection device called a pellicle. The pellicle prevents deposition of a foreign matter such as dust onto a reticle pattern surface, and suppresses the occurrence of defects caused by transfer of a foreign matter onto a wafer. FIG. 3 is a schematic view showing the structure of a pellicle.
A pellicle 24 is adhered to the pattern surface of a reticle 23 with an adhesive agent or the like. The pellicle 24 is made up of a support frame 25 large enough to surround the reticle pattern, and a pellicle film 26 which is adhered to one end face of the support frame 25 and transmits exposure light. If a space (to be referred to as a pellicle space hereinafter) defined by the pellicle 24 and reticle 23 is completely closed, the pellicle film may expand or contract due to the difference in atmospheric pressure between the inside and outside of the pellicle space or the difference in oxygen concentration. To prevent this, a vent hole 27 is formed in the support frame 25 so as to allow gas from flowing between the inside and outside of the pellicle space. An auto-screen filter (not shown) is attached to the vent hole 27 in order to prevent an external foreign matter from entering the pellicle space via the vent hole 27.
FIG. 4 is a schematic view showing an example of a reticle transfer path in the exposure apparatus shown in FIGS. 1 and 2.
In FIG. 4, reference numeral 22 denotes a foreign matter inspection device which measures the size and number of foreign matters such as dust deposited on the surface of the reticle 23 or pellicle film 26. The reticle 23 is loaded manually or by a transfer device (not shown) into the reticle load-lock chamber 13 serving as the entrance of the exposure apparatus. Since the reticle 23 and pellicle 24 are generally adhered outside the exposure apparatus, the pellicle 24 has already been adhered to the loaded reticle 23. The interior of the reticle load-lock chamber 13 is purged with inert gas until the interior reaches an inert gas atmosphere similarly to the housing 8. After that, the reticle 23 is transferred by the reticle hand 15 to any one of the reticle stage 1, reticle temporary stocker 18, and foreign matter inspection device 22.
As described above, an exposure apparatus using ultraviolet rays, particularly, an ArF excimer laser beam or fluorine (F2) excimer laser beam suffers large absorption by oxygen and moisture at the wavelength of the ArF excimer laser beam or fluorine (F2) excimer laser beam. To obtain a sufficient transmittance and stability of an ultraviolet ray, the oxygen and moisture concentrations are reduced and controlled strictly. For this purpose, a load-lock mechanism is arranged at a coupling portion between the inside and outside of the exposure apparatus. When a reticle or wafer is to be externally loaded, the interior of the exposure apparatus is temporarily shielded from outside air. After the impurity in the load-lock mechanism is purged with inert gas, the reticle or wafer is loaded into the exposure apparatus.
To ensure the transmittance and stability of fluorine (F2) excimer laser beam, the whole reticle stage (wafer stage) including the end face of a projection lens and a critical dimension measurement interference optical system is housed in an airtight chamber, and the interior of the chamber is purged with high-purity inert gas. In addition, the load-lock chamber is disposed adjacent to the airtight chamber in order to load/unload a wafer or reticle into/from the airtight chamber while maintaining a constant internal inert gas concentration. A reticle loaded into the load-lock chamber bears a pellicle. If the pellicle space defined by the pellicle, pellicle support frame, and reticle is not purged with inert gas, the transmittance and productivity decrease.
Japanese Patent Laid-Open No. 9-73167 discloses a technique of adhering a reticle and pellicle in advance in an inert gas atmosphere and filling the pellicle space with inert gas at an oxygen concentration of 1% or less. However, the transmittance of 157-nm light is merely 15% per cm in atmospheric-pressure gas at an oxygen concentration of 1%. At present, the air gap between the reticle and the pellicle film is about 6 mm. Even if this air gap is filled with gas at an oxygen concentration of 0.1%, the transmittance of 157-nm light at this air gap is merely 89.2%. The total space distance of an optical path from the light source of the exposure apparatus to a wafer exceeds at least 1 m. To ensure a transmittance of 80% or more in the 1-m space, the oxygen concentration must be suppressed to almost 10 ppmv/v or less, and ideally 1 ppm or less. In the pellicle space, the oxygen concentration must be 1 to 100 ppm or less in terms of the balance with another space and maintenance of the transmittance in the total space distance. This also applies to the moisture and carbon dioxide gas concentrations.
As a method of purging the pellicle space with inert gas, Japanese Patent Laid-Open No. 9-197652 discloses a technique of forming in a pellicle frame an opening for supplying or exhausting gas and a plug for sealing the opening, and filling the space defined by the pellicle frame, pellicle, and reticle with nitrogen in advance. The pellicle transmits oxygen, and when the reticle is left in an oxygen rich space for a long time, e.g., when the reticle is stocked, oxygen enters the pellicle space due to the difference in oxygen concentration from the outside, absorbing exposure light. Closing the pellicle space may deform and damage the pellicle owing to variations in atmospheric pressure.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide a technique of effectively purging a space almost closed with a master and pellicle film with inert gas in an exposure apparatus which uses ultraviolet rays as exposure light, purges the interior of the apparatus with inert gas, and projects the pattern of a master onto a photosensitive substrate via a projection optical system.
To overcome the conventional drawbacks and achieve the above object, according to the first aspect of the present invention, there is provided an inert gas purge method characterized in that a plurality of vent holes are formed in a structure obtained by surrounding by a surrounding member a gas purge space to be purged with inert gas, a vessel which forms a space around the structure is filled with the inert gas to cause the inert gas to enter the gas purge space, and the gas purge space is purged with the inert gas.
According to the second aspect of the present invention, there is provided an inert gas purge apparatus characterized in that a plurality of vent holes are formed in a structure obtained by surrounding by a surrounding member a gas purge space to be purged with inert gas, a vessel which forms a space around the structure is filled with the inert gas to cause the inert gas to enter the gas purge space, and the gas purge space is purged with the inert gas.
According to the third aspect of the present invention, there is provided an exposure apparatus characterized in that a space around a substrate is purged with inert gas by using the above-described inert gas purge apparatus, and a pattern is transferred onto the substrate.
According to the fourth aspect of the present invention, there is provided a reticle stocker characterized in that a reticle is stocked by using the above-described inert gas purge apparatus.
According to the fifth aspect of the present invention, there is provided a reticle inspection apparatus characterized in that a reticle is inspected by using the above-described inert gas purge apparatus.
According to the sixth aspect of the present invention, there is provided a reticle transfer box characterized in that a reticle is transferred by using the above-described inert gas purge apparatus.
According to the seventh aspect of the present invention, there is provided a device manufacturing method characterized by comprising the steps of applying a photosensitive material to a substrate, transferring a pattern onto the photosensitive material of the substrate coated with the photosensitive material by using the above-described exposure apparatus, and developing the substrate bearing the pattern.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part hereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.